Organochlorines such as the DDT metabolite DDE and polychlorinated biphenyls (PCBs) are persistent organic pollutants that may be toxic at levels encountered by the general population, through exposure in the food chain. PCBs interfere with neurodevelopment in animal experiments but human results are mixed. Each organochlorine has several types of biologic activity, so potential mechanisms of action are a challenge to pin down. For example, DDE interferes with androgen binding to its receptor. Most of my work in this area is based on analysis of serum from pregnant women who were in a cohort study named the Collaborative Perinatal Project (CPP). Here I describe some of my results from the CPP study. Results from other studies I have done on the health effects of persistent organic pollutants are indicated in the list of publications. Sera from over 50,000 pregnant women in the CPP have been frozen at -20 degrees Centigrade since 1959-1965, and with the health data on their offspring constitute a resource that is still actively used by many investigators. We selected a subset of about 2,800 children born in 1959-1966 and determined organochlorine concentrations in the corresponding stored maternal pregnancy sera. In earlier CPP papers we examined DDE levels in relation to fetal loss in previous pregnancies, and, in the CPP pregnancy, preterm birth, small-for-gestational-age, and birthweight; in all cases we found associations suggestive of adverse effects. To look for evidence of androgen-blocking, we examined maternal DDE level during pregnancy in relation to adjusted odds of cryptorchidism, hypospadias, and polythelia [extra nipples] among their male offspring; for these outcomes the results were suggestive but equivocal. Furthermore, we evaluated prenatal PCB exposure in relation to developmental assessment performance at 8 months of age, and hearing as reflected by audiometric examination at age 8 years. None of these studies showed any associations. Last year's progress Because this project is now in the data analysis and reporting phase, the progress is reflected by the publications. Briefly, in the past year, we have reported on: a) PCBs and performance on tests of cognitive performance (no association in our data), b)DDE and fetal loss (association found), c) DDE and body size in peripubertal males (no association found), d) PCBs and DDE in relation to menstrual cycle characteristics (some association with PCBs), e) PCBs and preterm birth and small-for-gestational age (no association found), f) PCBs and DDE and time to pregnancy (a suggestion of increased time to pregnancy with PCBs was found).